TM 1-411, Airplane Hydraulic Systems and Miscellaneous Equipment: 10 - Heating and Ventilating Equipment

SECTION X: HEATING AND VENTILATING EQUIPMENT

55. General.-Heating and ventilating of cockpits, cabins, and compartments of airplanes are accomplished by the use of hot-air systems, the source of heat being the exhaust manifold of the engine. In general, the systems may be classified as those in which the air is heated immediately at the exhaust manifold and those which employ an auxiliary steam system to transfer the heat from the exhaust stack to the cabin. The first type is generally used on the smaller type airplanes, e. g., attack, pursuit, and basic combat, while the larger airplanes usually employ the second and more elaborate heating and ventilating system.

56. Description.-a. In the simpler type system, a shell is mounted about a section of some part of the exhaust manifold and the hot air thus provided is led away through a tube, and admitted into the cockpit or cabin through a valve controlled by the pilot or passenger. Usually such control incorporates a means of directing the flow of incoming air to different parts of the cockpit or cabin at the discretion of the occupant. Ventilation is usually provided for by ventilating ports opening to the outside and equipped with controllable deflectors. Ventilating ports are so arranged that fresh air may be admitted direct to the cabin without going to the heater.

b. The heating and ventilating systems of the second type consist of two separate and distinct sections: an air-duct system which supplies, controls, and distributes the air used for heating and ventilating and a steam-generating system which serves to transfer the heat from the exhaust manifold to the incoming cold air. A typical system of this type is shown in figure 57.

c. An air-duct system is made up of the following:

(1) An air scoop opening forward into the air stream. It may be of the fixed or adjustable type.

(2) An induction or cold-air duct which conducts the air from the scoop to the radiator, and which in some cases includes a bypass duct around the radiator.

(3) A radiator in which the air is heated by passing over steamheated surfaces. (A more detailed description of this unit is included in d (3) below.)

(4) The distributor or warm-air ducts which conduct the heated air to the various compartments of the airplane.

(5) The air controls which control the rate of flow of the air in the various ducts. These may be either of the damper or the slidinggate type. In general, a cold-air control is contained in the induction duct to regulate the supply of cold air to the system and individual air controls are located in the separate distributor ducts to control or shut off the supply of hot air to the various compartments, e. g., cabin, cockpit or defroster. When a bypass around the radiator is used, an additional control is usually incorporated to regulate the relative amounts of incoming air going through and bypassing the radiator.

d. The steam generating system may be one of several types, each having a slightly different method of operation and control, but all depending on the same basic principles and having the same basic units. The units essential to all steam-generating systems are the water-supply tank or reservoir, the steam boiler, and the radiator or condenser. These units are described below in detail.

(1) The -water-supply tank or reservoir, as its name implies, is a unit used to contain the supply of water for the steam-generating system. It may be a sealed chamber designed to hold steam and water at pressures above atmospheric or merely a container with a filler neck to hold water at atmospheric pressure. In some installations a gage glass for water level and a pressure gage for tank pressures are included with this unit. Reservoirs are equipped with connections for piping to the other units used in the system and in some cases employ a drain cock.

(2) The boiler is a unit used to generate the steam by passing the exhaust gases from the engine over a number of seamless tubes containing water. It is located in the engine section of the airplane and in most cases forms a section of the exhaust-collector ring. Because of its high melting point and heat-resisting properties, inconel steel is used throughout in the construction of this unit. In general it consists of a circular tube casing containing a number of small seamless tubes manifolded internally at the top and bottom of the casing, so that there are only two external connections; a water inlet located at the bottom and a steam outlet located at the top. The construction of the several types used in these systems is similar, the principal difference being in the size and shape of the boiler casing and the number and size of the internal tubes. A condensate return from the steam duct to the water supply line may be internal or external to the boiler.

(3) The radiator or condenser is used to transfer the heat from the steam to the incoming cold air in the air system. It is the one unit which is common to both the air duct and steam generating systems and in general consists of a brass casing containing either a number of coiled copper tubes manifolded internally at the two ends of the unit, or a copper honeycomb. One end contains a fitting for connection to the steam duct from the boiler while the other end contains one or two fittings for connection to the water return system and in some cases a steam trap or water regulating valve. When a steam trap or water regulating valve is incorporated in the system, only water is allowed to pass from the radiator to the return lines and all the steam is condensed in the radiator. Fins of thin copper sheets run between the tubes to aid in the conduction of heat from the steam in the tubes to the incoming cold air passing through the casing.

e. Since, as previously mentioned, the, steam generating system may vary in details, the complete description of only one typical system will be presented here. The additional units included in this system and not previously described are as follows:

(1) A steam safety valve which is located on the steam duct. It is spring loaded and automatically limits the pressure in the system to a given value.

(2) A water shut-off valve which controls the amount of water passing into the boiler. The temperature of the warm air can be changed by allowing more or less water to run into the boiler.

(3) An auxiliary steam shut-off valve which is located forward and below the main shut-off valve. It is controlled by a push-pull rod and is of the butterfly type. This valve should be used only in case the relief valve in the bypass line fails to seat or does not function properly and allows the water to continue to run into the boiler after the main shut-off valve is closed.

(4) A water regulating valve or steam trap is located between the radiator and the water tank. It is a float type valve and allows the water to pass into the supply tank outlet line without the loss of any steam.

(5) A diaphragm type air valve located in the radiator return line is used to allow the elimination of air from the steam system. However, it will automatically close when the hot steam comes in contact with the diaphragm.

57. Operation.-The operation of this steam-heating system is as follows: When the water shut-off valve is opened, the water from the supply tank runs into the boiler tubes where the heat from the exhaust gases passing through the boiler casing converts it into steam. The steam is conducted through a steam duct to the multiple coils in the radiator, forcing the air in this part of the system through the diaphragm type air valve to the overboard vent line. The incoming cold air circulating about these coils absorbs heat from the steam which condenses and returns through the water regulating valve to the wa ter supply section of the system. The hot air passes from the radiator through the distribution or warm air ducts into the compartments of the airplane as determined by the various air controls. The safety relief valve controls the operating pressure of the steam system and opens at a predetermined value.

58. Maintenance.-At regular periods or when required, steam ducts are re-covered with approved materials. If the airplane is to remain idle and be subjected to freezing temperatures, the entire water system is drained. Precautions must be taken that none of the engine exhaust gases get into the cockpit or cabin through the heating and ventilating system., Small cracks may develop in the exhaust manifold in the section surrounded by the heater shell, which may allow poisonous gases to be drawn into the cockpit with the fresh air.